1. Field
The present method and device is directed generally to reducing cross-correlation false alarms in devices connecting to wireless networks, and more particularly, to a device and method for reducing cross-correlation false alarms in devices connecting to networks employing code division multiplexing, receiving satellite positioning system signals, and similar techniques.
2. Background
Many devices such as mobile stations (MS) and wireless base stations operate in wireless networks employing wireless communication protocols such as CDMA or operate employing satellite positioning system signals. These devices receive signals from various transmitting sources. These transmitting sources can present to the MS or the base station (wireless receiver) where they are to be individually detected and processed by the wireless receiver. Each of the transmitting sources code their transmission signal with an individual code to distinguish it from other signals from other transmitting sources. Each wireless receiver can then correlate the received signal from the transmitting source against a full or partial set of codes associated with wireless transmitters in that network. The wireless receiver will typically thereafter try to obtain a match for a particular code. When a match for a code is found, the transmitting source with the code is detected.
However, often times in the presence of noise and due to imperfect codes, matches in the correlation process from a transmitting source may occur even in the absence of a real signal or code at the wireless transceiver. When a wireless receiver determines a match in the search process that occurs based on noise at the wireless receiver without a real transmission from a transmitting source, the wireless reception is referred to as a cross-correlation false alarm.
For example, if a wireless receiver looking to receive a signal from a transmitting source having a code C1, a match in the correlation process may occur for some undesired signal having a code C2 in the absence of the desired signal having code C1. This match is due to a cross-correlation between code C1 and C2. Again, the resulting error is referred to as cross-correlation false alarm.
In the past, various techniques have been typically employed to reduce the occurrence of false alarms in wireless receivers, such as energy thresholds and redundant searches. These techniques have been used either individually or in combination to attempt to reduce the number of false alarms.
In the case of the energy threshold technique, the assumption is that the majority of false alarms due to noise or cross-correlation generally fall below some wireless signal energy level. Accordingly, the wireless receiver will typically only determine a match in the correlation process when the energy of the resulting correlation of the wireless signal that is received in the wireless receiver is above some predetermined energy threshold. A careful selection of the wireless signal energy threshold must be made so as to maximize the probability of detecting a real wireless transmission, such as real pilots at weak energy levels, while also minimizing the probability of detecting noise in cross-correlation false alarms.
Another known technique to address the cross-correlation false alarm problem is the use of redundant wireless signal searches. Redundant searches rely on the assumption that some independence exists between subsequent wireless signal searches. This independence typically decreases the probability of detecting false alarms with each additional wireless signal search.
For example, existing redundant search implementations allow for some independence between searches through time diversity. This relies on the assumption that noise is typically time-varying. Accordingly, searches at different points and times can typically be considered independent with respect to the noise. Thus, for typical noise functions, the probability of false alarms due to noise will decrease over time such as in an exponential fashion with subsequent searches.
However, it is been found that the energy threshold technique and the redundant search technique do not provide satisfactory prevention of cross-correlation false alarms. Cross-correlations are mainly a by-product of code properties and are not generally independent of time. Accordingly, there is a need for techniques that do not rely solely on time diversity or an energy threshold in order to benefit from cross-correlation independence.
Additionally, models for estimating the rate of cross-correlation false alarms described above in wireless networks may help in the design, development, and improvement of wireless networks. However, no current satisfactory model exists to estimate the rate of cross-correlation false alarms. Accordingly, there also is a need for an analytical model to help estimate the rate of false alarms from cross-correlation in wireless networks and more particularly, CDMA networks or satellite positioning systems.